A radiation curable composition, e.g. a radiation curable inkjet ink, typically comprises a photo-initiator. Upon exposure of said composition, the photo-initiator initiates the curing, i.e. the polymerization, of said composition.
Free radical photo-initiators can be classified as Norrish Type I or Norrish Type II photo-initiators. A Type I photo-initiator undergoes photo-cleavage to yield free radicals while a Type II photo-initiator produces free radicals through an abstraction process, in most cases hydrogen-abstraction. While a Type I photo-initiator produces free radicals through a unimolecular bond cleavage reaction, a Type II photo-initiator forms free radicals in the presence of a second molecule, the so called co-initiator also referred to as polymerization synergist.
A first problem for free-radical polymerization processes is the inhibition of the polymerization, i.e. decrease of the curing speed, by oxygen. Inhibition by oxygen results not only in a decreased overall curing speed, but can also result in an inhomogeneous curing, e.g. poor surface curing resulting in tacky surfaces.
Amines are well known in the art as co-initiators for Type II photo-initiators and to suppress oxygen inhibition of free radical polymerization processes. The role of amines in curing systems has been reviewed, for example, by R. S. Davidson in “Radiation Curing in Polymer Science and Technology, Volume III”, edited by J. P. Fouassier et al., Elsevier Applied Science, 1993, pages 153-176.
A second problem, associated with co-initiators, e.g. amines, present in a radiation curable composition, may arise when unreacted co-initiator remains in the cured composition. Hydrogen transfer from an amine co-initiator to a Type II photo-initiator is rarely quantitative. The unreacted co-initiator remains mobile in the cured composition and may adversely affect the physical properties of the cured composition or may diffuse out of the cured composition. If the radiation curable composition is printed upon food packaging, extraction of the unreacted co-initiator, also referred to as extractables, into the food may cause health risks. One way to obtain less extractables may be the improvement of the reactivity of the co-initiators, in order to avoid unreacted co-initiators after curing. A decrease of the diffusion out of unreacted co-initiators may be another way to provide less extractables. Decreasing the diffusion out may be realized by providing the co-initiators with a polymerizable group in order to co-polymerize the co-initiator upon curing or by increasing the molecular weight of the co-initiators.
U.S. Pat. Nos. 3,845,056, 3,963,771 and 4,045,416 disclose amine acrylates, produced by the reaction of a polyacrylate ester, preferably a diacrylate ester, with an amine having at least one hydrogen attached to the nitrogen atom. Said amine acrylates, comprising polymerizable groups, provide a fast curing speed to radiation curable coating compositions.
EP-A 1 147 098 discloses multi-functional reactive amine acrylates having a low viscosity, prepared by the reaction of multifunctional (meth)acrylates with cyclic secondary amines. These amine acrylates provide a good curing speed, a higher thermal stability and less extractables to radiation curable compositions.
In WO 03/091288 a new class of amine co-initiators, in combination with Type I and/or Type II photo-initiators, is disclosed, said amine co-initiators being trialkylamines having a total of 10 to about 36 carbon atoms, wherein at least one alkyl group has a chain length of at least 8 carbon atoms. According to WO 03/091288, said amine co-initiators, when used in combination with a Type II photo-initiator, provide less extractables after curing compared to conventional amines as e.g. N-methyl-N,N-diethanolamine.
WO 99/07746 discloses a radiation curable composition containing a radiation curable resin, a photo-initiator and an amine as co-initiator, characterized in that said amine is a compound containing at least one tertiary amino group and wherein at least one substituent of the tertiary amino group is an aliphatic chain containing at least one electron-withdrawing group. Said amine compound is preferably a branched, highly branched or star-shaped dendrimer comprising at least one tertiary amino group. Said co-initiators, in combination with Type II photo-initiators provide a higher curing speed and less extractables.
EP-A 1 616 922 discloses a radiation curable composition containing a radiation curable resin, a photo-initiator and a co-initiator characterized in that said co-initiator comprises a dendritic polymer core with at least one co-initiating functional group, e.g. aliphatic or aromatic amines, as an end group on the polymer core. The radiation curable compositions according to EP-A 1 616 922 are characterized by a high curing speed and a low amount of extractables.
Increasing the molecular weight of the co-initiator, to decrease the amount of extractables, may however result in a too high viscosity of the curable composition.
Other problems associated with curable compositions, in particular inkjet inks, are photoyellowing and adhesion.
Photoyellowing is a discoloration effect seen after curing due to decomposition of photoinitiators and/or co-initiators. This can be especially well observed for cyan and white radiation curable inks containing large amounts of isopropylthioxanthone type photoinitiators, which after printing and curing result in a greenish cyan respectively a yellowish white colour. EP-A 0 036 075 and DE-A 3 126 433 disclose the use of specific mixtures of photoinitiators in the photopolymerisation of ethylenically unsaturated compounds to obtain polymers characterized by extremely low levels of yellowing.
The behaviour and interaction of a UV-curable ink on a substantially non-absorbing ink-receiver was found to be quite complicated compared to water-based inks on absorbent ink-receivers. In particular, a good and controlled spreading of the ink on the ink-receiver proved to be problematic and adhesion problems were observed on using different types of non-absorbing ink-receivers. One way to approach these problems is to develop and use different ink sets for different types of substrates, but this is a not a preferred solution since changing inks in the printer and print head is very time consuming and not really a viable solution for an industrial printing environment. The adhesion may be influenced by using different polymerizable compounds, surfactants, binders and/or organic solvents. U.S. Pat. No. 6,814,791 discloses inkjet printing methods wherein the ink composition comprising methyl acetate as a solvent is printed upon substrates of propylene and ethylene. The use of a well-chosen solvent usually results in partial swelling or dissolution of the substrate surface which leads to better adhesion, but can also cause problems of blocked nozzles in the printhead due to evaporation of solvent. It is known that the adhesion of radiation curable inks can also be promoted on polyvinyl chloride substrates when one or more monomers are used that are suitable for the swelling of the PVC substrate and which are selected from the group consisting of tetrahydrofurfuryl acrylate, 1,6-hexanediol diacrylate and N-vinyl caprolactam. However, adhesion on polycarbonate substrates is promoted when one or more monomers are used that are suitable for the swelling of the polycarbonate substrate and which are selected from the group consisting of 2-phenoxylethyl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate and polyethyleneglycol diacrylate. As a consequence one has to make the “best possible” mixture of monomers suitable for both the swelling of polyvinyl chloride substrates and polycarbonate substrates. Often such a compromise, whereby acceptable adhesion is obtained on several ink-receivers by making a complex mixture of ingredients, has a negative effect on the dispersion stability of a pigmented inkjet ink. Adhesion problems have also been associated with shrinkage of an ink-layer after radiation curing. In this aspect, cationic inks have been regarded to be superior in comparison to free radical polymerizable inks. EP-A 1 705 229 discloses cationically polymerizable inkjet inks exhibiting good adhesion and storage stability. U.S. Pat. No. 6,310,115 discloses radiation curable inkjet ink compositions comprising radiation curable monomers containing vinylether and acrylate functions, which can be cured both by cationic polymerization and free radical polymerization. In free radical inkjet inks, high amounts of monofunctional acrylates are thought to be advantageous for adhesion. Both EP-A 1 668 084 and U.S. Pat. No. 7,104,642 address adhesion and disclose discloses radiation curable inkjet inks comprising monofunctional acrylate compounds in amounts of 65% by mass or more. Instead of adapting the inkjet inks, it has become the general approach to modify the surface chemistry of the ink-receiver either by a pre-treatment such as plasma or corona treatment or by applying a suitable surface layer, a so-called primer. Corona discharge treatment and plasma treatment increase the cost, complexity and maintenance of the equipment used to process the substrates. Substrates may contain significant impurities or irregularities that may interfere with the treatment of the substrate, and hence not result to the uniform spreading and adhesion of ink. The other possibility is the application of a primer prior to jetting the inkjet inks. Generally, the surface layer is coated and dried or cured before jetting the inkjet ink as, for example, in the inkjet printing process in EP-A 1 671 805 and US 2003 021 961, but it can also remain a wet, un-cured surface layer as in WO 00/30856.